Impedance Plot 288B LMS

[OldBlindJim]

Howdy All,

I'm ready to start LMS impedance ploting the first group of 288-Bs.

Does anyone have a LMS plot of the 288-B for reference?
Was it in open air, tubed or still on the flare?

Thanks
OB Jim

[Earl K]

Hi ,

- Here's how Altec showed off the 288-8G ( to it's best ) .
- These are exceptionally good looking curves / by any standard ( old or new ) .

- If I were you, I'd do something similar if you want to show off the performance of your cache of 288B(s) .

- ie;
(i) Freq. Response on a large 220hz horn ( your largest multicell ) / plus on a 2" PWT ( if you have the means to construct a Plain Wave Tube ) .

(ii) Impedance ; as above ( your largest multicell ) / & on a 2" PWT ( if you can construct one ) .


:)

[OldBlindJim]

Hi ,

- Here's how Altec showed off the 288-8G ( to it's best ) .
- These are exceptionally good looking curves / by any standard ( old or new ) .

- If I were you, I'd do something similar if you want to show off the performance of your cache of 288B(s) .

- ie;
(i) Freq. Response on a large 220hz horn ( your largest multicell ) / plus on a 2" PWT ( if you have the means to construct a Plain Wave Tube ) .

(ii) Impedance ; as above ( your largest multicell ) / & on a 2" PWT ( if you can construct one ) .


:)
Thanks, good input.

Here's what I'll do. I'll setup a bench with a 15 cell and a single throat. Mount the driver and just sweep the impedance. We all are pretty much at agreement of there performance so I'll leave out the curve.

I'll Sweep first, then crack open the egg to see if the yoke is damaged.

Write down all the info I can find off the driver along with pictures.

Sound OK to everyone?

Old Blind Jim

[Earl K]

Here's what I'll do. I'll setup a bench with a 15 cell and a single throat. Mount the driver and just sweep the impedance. We all are pretty much at agreement of there performance so I'll leave out the curve.

I'll Sweep first, then crack open the egg to see if the yoke is damaged.

Write down all the info I can find off the driver along with pictures.

Sound OK to everyone?

No .

- Post both an Impedance Curve and a Frequency Reponse curve .
- Doing so will determine the relative health of the diaphragm .

- Here's what the FR of a "pooched" 288-8K diaphragm can look like after it has been used for some time . That's the first pic . ( ignore all the HF hash over 16K ).
- The second pic is the same 288-8K, now with a new GPA diaphragm .
- The horn used was an Altec multicell .
- ( pics courtesy of sfogg, ie ; Shawn )


:o:

[OldBlindJim]

No .

- Post both an Impedance Curve and a Frequency Reponse curve .
- Doing so will determine the relative health of the diaphragm .

- Here's what the FR of a "pooched" 288-8K diaphragm can look like after it has been used for some time . That's the first pic . ( ignore all the HF hash over 16K ).
- The second pic is the 288-8K with a new GPA diaphragm .
- The horn was an Altec multicell .
- ( pics courtesy of sfogg ; Shawn )


:o:

OK,
I'll be using LMS DOS version.
1 meter at 2.83 volts On Center be OK?
6 of the are 288-B yellow gray label so they will show a lot of time on them.
4 are much newer but with no labels (flaked out) 1968 or 58 not sure. last three digits are 842.

OBJ

[Earl K]

OK,
I'll be using LMS DOS version.
1 meter at 2.83 volts On Center be OK?

- That'll be deafening and IMO, quite unnecessary .
- Afterall, you're not really trying to re-establish Altecs' sensitivety specs. here.
- A good FR plot tells people a lot . Good FR response just won't happen with a fatigued diaphragm +/or a weakened alnico magnet .
- I'd limit the input voltage to "what ever seems loud enough" to get an accurate FR reading . That could easily be a fraction of a volt .



:)

[OldBlindJim]

- That'll be deafening and IMO, quite unnecessary .
- Afterall, you're not really trying to re-establish Altecs' sensitivety specs. here.
- A good FR plot tells people a lot . Good FR response just won't happen with a fatigued diaphragm +/or a weakened alnico magnet .
- I'd limit the input voltage to "what ever seems loud enough" to get an accurate FR reading . That could easily be a fraction of a volt .



:)

OK,
I'm in the middle of first runs right now.
Will check back in tomorrow (sat).

OBJ

[OldBlindJim]

I've done only the imp sweeps and tomorrow I'll do FR.

http://home.earthlink.net/%7Ejvanhornrlh/Altec/288B_1st_2nd.jpg (http://home.earthlink.net/%7Ejvanhornrlh/Altec/288B_1st_2nd.pdf)

Here a link to the PDF file.


Both came off the same "Y" throat then mounted onto a single with the 15 cell and swapped in our warehouse at 1 watt.

# 1 metered @ 13.6 ohms
# 2 metered @ 13.1 ohms

Possible magnet issues?

Both come blasting in at 12K (I could feel it more then hear) so it will be interesting to see the FR.

OBJ

[OldBlindJim]

OK
36" @ 2.83Vrms
Now keep in mind I haven't had this rig out for YEARS and I just did a rough setup. (Mic, masking tape and a tripod)
Here is link to the PDF file for the first 4 288b.

http://home.earthlink.net/%7Ejvanhornrlh/Altec/288BFR14.jpg (http://home.earthlink.net/%7Ejvanhornrlh/Altec/288BFR14.pdf)

Looks like #2 needs a re-mag!
Metered the same as the others.

Be back later with more
OB Jim

[Zilch]

I'm having difficulty with the horizontal scale. Where it says "5k" at the bottom, there, that's 1 kHz, no?

And the rolloff begins at 2 kHz?

Down ~10 dB at 10 kHz, and 20 dB at 20 kHz, the best-extending two?

[OldBlindJim]

I'm having difficulty with the horizontal scale. Where it says "5k" at the bottom, there, that's 1 kHz, no?

And the rolloff begins at 2 kHz?

Down ~10 dB at 10 kHz, and 20 dB at 20 kHz, the best-extending two?

1. I fixed the PDF issue. Old Adobe! Should read correctly now.
2. I'm in a warehouse and not a lab and the drivers are 50 years old.
Other issue may give at fault. Placement, Mic cal, Me. What I'm looking for is if they are working so all the measurement are done from the same place.
3. Driver two may have a rubbing voice coil. I cracked open the egg and haven't been able to find the rub.

OB Jim

[Zilch]

Thank you Jim, I understand.

Please know I am not being critical; just needed help in reading the results, is all.... :thmbsup:

[OldBlindJim]

OK here's the imp plots.

http://home.earthlink.net/%7Ejvanhornrlh/Altec/288B-IMP.jpg (http://home.earthlink.net/%7Ejvanhornrlh/Altec/288B-IMP.pdf)

#2 must have a rub. I'll get a gauge set and see if it's the gap. Looks good though.

The other may have just magnet issues.

OB Jim

[OldBlindJim]

Thank you Jim, I understand.

Please know I am not being critical; just needed help in reading the results, is all.... :thmbsup:

No Problem:D

[moldyoldy]

...snip...Possible magnet issues?

I'd suspect a crack in #2 'fram surround, which can sound like a coil rub. A weak motor'd look more down overall instead of the big hole on the bottom end. Easy enough to verify with a 'fram swap.

[OldBlindJim]

I'd suspect a crack in #2 'fram surround, which can sound like a coil rub. A weak motor'd look more down overall instead of the big hole on the bottom end. Easy enough to verify with a 'fram swap.

I'll do a diaphragm swap tomorrow and give it another try.

It's Sunday and I gotta go Easter Outing.
Eat Drink....Burp!:blink: Later all

OB Jim

[Steve Schell]

The low end rolloff on #2 looks like a voice coil rub to me. The diaphragm is held captive by the friction and can't move like it should. Above a few kiloHertz the excursions are so tiny that it measures normally.

The other response plots look fine for a 288 on a multicellular horn. The highs are dispersed widely by the horn, so the response looks similar to the power response- no on axis beaming to flatter an on axis measurement. As true for most any midrange compression driver, the mass corner is about 2 or 3kHz. and the power resonse falls off at 6dB/octave from there.

Nice looking LMS plots, OBJ. I recently gave away an old dos LMS setup, but its graphs never looked that nice. I'd be tempted to go easy on the input power; I most often test compression drivers at .1W, or even less on a plane wave tube. In service these devices were being fed milliwatts most of the time.

[Earl K]

The other response plots look fine for a 288 on a multicellular horn. The highs are dispersed widely by the horn, so the response looks similar to the power response- no on axis beaming to flatter an on axis measurement. As true for most any midrange compression driver, the mass corner is about 2 or 3kHz. and the power resonse falls off at 6dB/octave from there.

- An intriguing diagnosis, I was developing a somewhat different conclusion ( about the health of those diaphragms ) based on info gleaned from Todds' Altec site . The thread in question is called 288-8K Extension (http://www.hostboard.com/cgi-bin/ultimatebb.cgi?ubb=get_topic&f=3729&t=1792)

- I guess the question can become ; Does measuring a multicell with @ 1/2 as many cells, from a much closer range , result in so much more apparent HF output ?

[Steve Schell]

Hi Earl,

I suppose a narrower pattern horn might change things a little, although the entire passband (except maybe on the low end where the horn is starting to lose pattern) would be boosted a similar amount due the same energy confined to a smaller area.

These lousy looking high frequency responses are familiar to me as I frequently use conical (straight sided) horns. With these there is very little h.f. beaming; only that which takes place within the driver, really. So, the response curve measured pretty much anywhere within the pattern of the horn closely resembles the driver's power response. Plane wave curves look this way too, except that the low end is exaggerated mightily by the heavy loading.

I followed your link. In the first curve a LeCleach horn is used, which is a curved wall horn that will progressively beam high frequencies. Even so, the response starts falling gently about 3kHz., ignoring the little peaks and the big one centered on 13kHz. which is probably a major suspension breakup. Not sure what to think about the bar graphs further down, but I certainly prefer the second one!